The floating-gate transistor in non-volatile memory is a well known form of electrically erasable programmable read-only memory, or EEPROM. EEPROM devices generally include a grid or array of transistors having one or more floating gates protected by a high-quality insulator. By application of higher-than-normal electrical potentials to terminals of the EEPROM transistors, electrons become trapped on the floating gate, thereby changing the voltage at which the transistor turns on. EEPROM transistors can be reset or erased to a base state (all “1”s or “0”s, depending on the design), either electrically or optically (e.g., by applying ultraviolet [UV] light). Once packaged, the EEPROM device generally cannot be reset or erased with UV light. However, the electrical erase process has the disadvantage of degrading the charge storage capability of the device, so memory systems based on floating-gate transistors generally have finite lifetimes, on the order of 105 write operations.
Flash memory is substantially identical to EEPROM, and differs largely in internal layout and/or processing logic. Flash memory is generally written and erased in blocks, rows or columns, which greatly simplifies the internal wiring and allows for higher densities. Areal density (i.e., memory storage bits per unit area) is the main determinant of cost in most memory systems, and due to this, flash memory has evolved into one of the lowest cost solid-state memory devices available.